Method for depositing metals on nonconducting substrates



y 1961 F. A. REISS 2,986,480

METHOD FOR DEPOSITING METALS ON NONCONDUCTING SUBSTRATES Filed May 18,1960 INVENTOR. FELIX A. REIS BY WW ATTOR.NEY$

United States Patent IVIETHOD FOR DEPOSITING METALS ON NON- i CONDUCTINGSUBSTRATES Felix A. Reiss, Ann Arbor, Mich., assignor to the UnitedStates of America as represented by the Secretary of the Air ForcevFiled May 18, 196 0, Ser. N0.'30,038

3 Claims. (Cl. 117-227) This invention relates generally to coatingnonconducting substrates and, more particularly, to methods andapparatus for applying a uniform metal coating to a nonconductingsubstrate by chemical reduction.

Various methods of the prior art for coating nonconducting substrateshave proved to be inadequate due to disadvantages inherent in suchmethods. For example, if the coating of fibrous glass yarn with a silversolution were carried out in a tank or vessel of any shape or size, thebulk of the silver deposition would take place on the walls of thecontaining vessel and only a minimal fraction of the available silver,if any would deposit on the glass fiber. Shibsequent methods eliminatedthe use of a containing vessel by applying, drop by drop, a silversolution and a reducing solution, respectively, to afibrous glass strandadvancing horizontally underneath the two solutions. Such methods wereseverely limited by the relatively slow rate of speed at which it wasnecessary to advance the fibrous glass and. were further characterizedby the relatively nonuniform electro-conductivity of the coated productproduced thereby.

It is, therefore, an object of the present invention to provide a methodfor depositing metals on nonconductingsubstrates.

It is a further object of the present invention to provide a method forapplying a uniform coating of silver to fibrous glass yarn.

It is another object of the present invention to provide a method andapparatus for rapidly and continuously applying a uniform coating ofsilver to a nonconducting substrate.

A still further object of the present invention is to provide a methodand apparatus for depositing coherent and adherent silver films onnonconducting filaments or yarns at fairly high speeds with a minimumloss of silver.

The novel features characteristic of the present invention, as well asadditional objects and advantages thereof, will be better understood inthe following detailed description of the certain form of apparatus forcarrying out the process, when read in connection with the accompanyingdrawings in which:

Fig. l is a view of apparatus for coating a fibrous glass yarn withsilver.

Referring now to Fig. 1, a fibrous glass yarn 1 is unwound from apackage 2 and passes through conventional pretreatment equipment,indicated generally at 9, where the yarn undergoes conventionalpretreating processes, such as wetting, sensitizing, etc. As the fibrousglass yarn 1 emerges from the pretreatment equipment 9, it enters anexcess liquid collection tank 3 by means of idler rolls 7. Fromcollection tank 3, the yarn rises perpendicularly to another idler roll7 and continues on to pass through conventional and drying equipment,indicated generally at 10. The yarn is then wound onto a takeup drum 15,which is revolving in the direction of the arrow shown thereon to pullthe yarn through the entire aforementioned process. A pair of reactantcontainers, 11 and 12, respectively, are positioned adjacent the upperidler roll 7 and each is pro- "ice vided with a metering valve 13 formetering the reactant contained therein, as will be hereinafterdescribed.

In operation, reactant container 11 is supplied with an aqueous silvercomplex solution comprising 120 grams of silver nitrate andapproximately 30% ammonium hydroxide to dissolve the brown precipitateformed thereby, plus a sufiicient amount of water to make up one liter?of: solu-v tion. Reactant container 12 is supplied with an aqueousreducing solution consisting of 20 gramsof hydrazine, 45 grams of sugarand 5 grams of gelatin plus a suflicient amount of water to make oneliter. of solution. The sugar and gelatin are added to the reducingsolution to promote the mechanism of droplet. formation, aswill behereinafter described. As the fibrous glass strand is, advancedperpendicularly upward to a height of. approximately 50 feet, as shownby the arrow Y, at a speed of approximately 100 ft./min., the silvercomplex solution and the reducing solution are metered drop by drop fromreactant containers 11 and 12, respectively, by. means of meteringvalves 13. Metering valves: 13: are positioned with respect to theadvancing fibrous glass. yarn 1 sov that the droplets of the respectivesoltuion beingmetered therefrom contact and slide downwardly along theupwardly advancing yarn in the diretcion shown by the arrow L. Noattempt is made to bring aboutmixing of the respective solutions at thepoint of application, but rather it has been proved in practiceadvantageous to space the points of application severalinchesv apart.

The mixing of the two solutions and the consequent silvering of theupwardly. advancing fibrous glass yarn takes place while the solutionsand yarn'move: in opposite directions. It is this perpendicularly:opposed motion of the substrate and solutions that induces substantiallyincreased capillary action between the globules of the respectivesolutions in the, interstices of the fibrous glass yarn. Y.

It has been discovered that the initial drops or globules of each of thesolutions, upon coming into contact with the fast moving yarn, break upinto a series of uniformly spaced small droplets that seem to travelwith the yarn. The next drop of one of the two solutions coming downalong the yarn is very large relative to the clinging drop lets andswallows on its way down the yarn one droplet after another, therebyincreasing its size as it travels downwardly. When this oversize globulereaches a certain volume, it begins to spout a tail, which seems tocling to the yarn more tenaciously than the spherical drop. Theformation of a tail on a downwardly falling drop is further induced bythe force of gravity operating on the drop. The tail section of the dropis further elongated by the oppositely moving yarn. At this point in thesequence of events, the action of the downwardly traveling drop with itselongated tail section is difficult to predict. Sometimes the tailsection consumes the drop entirely; sometimes the tail section breaksoff to release a medium sized drop that continues to travel downwardlypicking up droplets, and further growing and dividing. The tail sectionclings to the yarn like a thin sheath of liquid and, as it reaches acertain length, it breaks up into many small droplets that are picked upagain by the next large drop of either solution coming down from above.This sequence of events, though well defined with respect to themechanics involved, is entirely random with respect to the chemicalcomposition of the droplets and any individual large drop. Since thisphenomenon repeats itself over the entire fifty feet of yarn, theresulting mixing of the silver complex solution and the reducingsolution is complete and uniform. The excess liquid from this process iscaught in the collection tank 3 and is subsequently drained therefrom bymeans of a faucet 5.

When the silvering process is completed, the reaction products, whichare a mixture of silver, silver hydroxide conditioned yarn, but showvery low or no electrical conductivity. However, when they are heated inopen air to about 600 F. for about one ortwo seconds,the'y change intoastrongly adherent coating of very good conductivity; the change inconductivitybeing accompanied by a change in color from black to ayellowish-brown. The silver coated yarn is brought to a bright metallicsilver luster by washing the yarn in water. The conditioning orsensitizing of the fibrous glass yarn prior to the silvering process isessential for a successful operation, and is accomplished by pulling theyarn through a strongly acidic solution, for example, hydrochloric acid,of stannous chloride and subsequent washing in tap water. The fibrousglass yarn is thus exposed to .theaction f the stannous chloridesolution for about one or two seconds to remove all inhibitingimpurities, such as sizing preparations used in the yarn making process,from the surface of the glass fibers.

By means of the process of the present invention, it is possible toapply a continuous and uniform coating of silver to a nonconductingsubstrate, such as fibrous glass yarn, at a speed more than three timesthat disclosed by the prior art and with aminimum. loss of silver. Sincethe silvering process repeats itself over a fifty-foot span of yarn, theglobules of the silvercomplex solution and the reducingsolution arethoroughly and uniformly, distributed throughout the interstices of thefibrous glass yarn. The reduction reactionis completed by the application ofheat, as previously described. The resulting silver coated yarnis uniformly electroconductive, which, in addition to its decorativeefiect, can be used in any situation demanding such a product. 1

It will be readily apparent to persons skilled in the art that metalsother than silver, such as gold, copper, mercury, etc., and .anynonconducting substrate in addition to fibrous glass yarn, such asnylon, can be used without departing from, the spirit of the presentinvention.

Although the process of the present invention hasbeen .described withrespect to a specific embodiment, it should be understood that obviousmodifications may be made within the spirit and scope of the appendedclaims.

I claim:

1. A method for depositing coherent and adherent silver films onnonconducting yarns at fairly high speeds with a minimum loss of silver,comprising continuously advancing the yarn upwardly in a substantiallyvertical path at a speed of approximately lGOfeet per minute to a heightof approximately fifty feet, dropping a globule of silver complexsolution from a first applicator, directing the downwardly fallingglobule of silver complex solution to contact the upwardly advancingyarn, dropping a globule of a reducing solution from a secondapplicator, directing the downwardly falling globule of reducingsolution to contact the upwardly advancing yarn, allowing the globulesof the silver complex solution and the globule of the reducing solutionto slide downwardly along the upwardly advancing yarn for mixing the twosolutions and distributing the mixture throughout the interstices of theyarn by capillary action, and allowing the reduction reaction to:proceed to completion to effect a substantially continuous deposit ofsilver upon the surface of the yarn.

' 2. The method of claim 1 wherein the silver complex solution comprises120 grams silver nitrate per liter and an amount of ammonia suflicientto dissolve the precipitate formed by the reduction reaction and thereducing solution comprises 20 grams of hydrazine, 45 grams of sugar and5 grams of gelatin per liter.

3. The method of claim 1 wherein the silver coated yarn is subsequentlyheated in open air to approximately 600 F. for approximately l-2 secondsto substantially improve the electrical conductivity thereof.

References Cited in the file of this patent

1. A METHOD FOR DEPOSITING COHERENT AND ADHERENT SILVER FILMS ONNONCONDUCTING YARNS AT FAIRLY HIGH SPEEDS WITH A MINIMUM LOSS OF SILVER,COMPRISING CONTINUOUSLY ADVANCING THE YARN UPWARDLY IN A SUBSTANTIALLYVERTICAL PATH AT A SPEED OF APPROXIMATELY 100 FEET PER MINUTE TO AHEIGHT OF APPROXIMATELY FIFTY FEET, DROPPING A GLOBULE OF SILVER COMPLEXSOLUTION FROM A FIRST APPLICATOR, DIRECTING THE DOWNWARDLY FALLINGGLOBULE OF SILVER COMPLEX SOLUTION TO CONTACT THE UPWARDLY ADVANCISINGYARN, DROPPING A GLOBULE OF A REDUCING SOLUTION FROM A SECONDAPPLICATOR, DIRECTING THE DOWNWARDLY FALLING GLOBULE OF REDUCINGSOLUTION TO CONTACT THE UPWARDLY ADVANCING YARN, ALLOWING THE GLOBULESOF THE SILVER COMPLEX SOLUTION AND THE GLOBULE OF THE REDUCING SOLUTIONTO SLIDE DOWNWARDLY ALONG THE UPWARDLY ADVANCING YARN FOR MIXING THE TWOSOLUTIONS AND DISTRIBUTING THE MIXTURE THROUGHOUT THE INTERSTICES OF THEYARN BY CAPILLARY ACTION, AND ALLOWING THE REDUCTION REACTION TO PROCEEDTO COMPLETION TO EFFECT A SUBSTANTIALLY CONTINUOUS DEPOSIT OF SILVERUPON THE SURFACE OF THE YARN.